The Astronomers’ Blessing

In the Jewish liturgy every morning we say the blessing Yotzer Or, “Who creates (or ‘forms’) light”, the first of two brachot (blessings) before reading the Shema.

I like to think of this as “the astronomers’ blessing” (or at least one of them…) because it focuses on things near and dear to astronomers, in particular light, but as we shall see, also much more. Personally, I find it to be very helpful and meaningful to think about these astronomical ideas when praying every morning; they provide a good booster for my kavana (“focus”, “intention” or “mindfulness” on the meaning of the prayer).

Blessed are you, LORD our God, King of the universe, who forms light and creates darkness, who makes peace and creates all things.

Note that this blessing is based on the verse in Isaiah 45:7:

.יוצר אור ובורא חשך, עשה שלום ובורא רע; אני ה’, עשה כל-אלה

I form the light, and create darkness; I make peace, and create evil; I am the LORD, that doeth all these things.

However, when composing the blessing, the rabbis of the Talmud rephrased part of it to focus more on the creation of the entire universe and less on the idea that God created evil – to focus on cosmology rather than theodicy. We find this discussion in the Talmud:

MISHNAH. IN THE MORNING TWO BLESSINGS ARE TO BE SAID BEFORE IT AND ONE AFTER IT. IN THE EVENING TWO ARE SAID BEFORE IT AND TWO AFTER IT…

GEMARA. What benedictions does one say [in the morning]? R. Jacob said in the name of R. Oshaia: (11b) ‘[Blessed art Thou] who formest light and createst darkness’. Let him say rather: ‘Who formest light and createst brightness’? — We keep the language of the Scripture. If that is so, [what of the next words in the text], Who makest peace and createst evil: do we repeat them as they are written? It is written ‘evil’ and we say ‘all things’ as a euphemism. Then here too let us say ‘brightness’ as a euphemism! — In fact, replied Raba, it is in order to mention the distinctive feature of the day in the night-time and the distinctive feature of the night in the day-time.

In cosmology, the matter in universe formed from that initial light, and so, essentially, when we describe and bless God as creator of light, it is implicitly saying He is the creator of everything, which the blessing immediately reiterates (for the benefit of those who are not familiar with modern cosmology!)

However, there is more to it than that. The blessing continues וּבוֹרֵא חֹשֶׁךְ – and creates darkness.

Darkness was also created by God, and is not merely the absence of light, as we might think from our frequent experience of darkness. In the cosmological context there are (at least) two possible ways to understand this phrase in the blessing.

One possible explanation refers to the early Universe as explained by Big Bang cosmology. For hundreds of thousands of years after the initial light of the Big Bang, the Universe was ”dark”. During this era of the early Universe – which followed shortly after the light of the Big bang – the Universe was filled with free (i.e. not bound to atomic nuclei) electrons. Due to these electrons bouncing around everywhere, light could not propagate (very far) as it would “hit” and “bounce off” of electrons after a very short distance. It was “hidden” everywhere, but it could not “shine forth”, thus making darkness. After about 300,000 years, electrons became bound to atomic nuclei (of hydrogen and helium) and formed complete atoms. This change enabled light to travel freely through Universe.

Transition from era of opacity to light traveling freely

This then could be an interpretation of the phrase immediately following “u’boreh choshech” – “oseh shalom u’boreh et hakol”: Shalom meaning wholeness – from the word “shalem”. “Whole” referring to the combination of electrons and nuclei into whole atoms. This explanation views the phrase as a chronological order.

Another possible way to understand these phrases is referring to dark matter and dark energy – which comprise the majority of the Universe!

What are dark matter and dark energy, you ask?

Several different types of astronomical observations have led astronomers to conclude that there must be much more mass than is visible – or “luminous mass” – in various situations throughout the Universe. The gravitational pull on visible objects exerted by something that is not luminous leads astronomers to conclude that there must be unseen mass involved. This unseen mass is called “Dark Matter” since it either does not interact with light at all, or at least does not emit or reflect any that can be observed.

There are several types of observations that indicate the existence of dark matter:

Spiral galaxies, including our own Milky Way, have rotation curves that indicate the majority of their mass is beyond the visible edge of the galaxy. A rotation curve is a graph of the speed of rotation vs. the distance from the center. If the majority of mass were concentrated at the center, as in the solar system, for example, the speed of rotating bodies gets slower the farther out from the center a mass is. However, observations of galactic rotation curves of spiral galaxies – made by comparing doppler shifts, that correspond to speed differences, on opposite sides of the galaxy – show the rotation speeds to remain roughly flat out to the edge of the observable (i.e. visible) part of the galaxies. This indicates that there must be a lot more mass beyond the visible edge of the galaxies.

Diagram of observations of galactic rotation speeds using doppler shift of opposite sides of a galaxy.Rotation curves of several galaxies, with orbital velocity plotted vs. distance from center of galaxies.Diagram of spiral galaxy showing luminous matter surrounded by much larger halo of dark matter. (from Pearson, Cosmic Perspective.)

A similar phenomenon is observed for elliptical galaxies, although by a different method.

Similarly, observations of the rotation speeds of individual galaxies within galaxy clusters, also using doppler shifts, show that there is about 50 times more mass in the cluster than is visible in stars!

Clusters of galaxies have hot gas between the galaxies, so hot that the gas emits high energy X-rays. The temperature – a measure of the average kinetic energy of particles – of this gas is primarily determined by the mass within the cluster. Observations of X-ray emitting cluster gas also indicate that there must be unseen mass in clusters. The proportion of clusters’ mass is 2% stars, 13% hot gas and 85% dark matter!

Yet another method of measuring the mass of galaxy clusters also indicates that they are more massive than their luminosity would indicate. Einstein’s Theory of General Relativity explains that even light experiences the pull of gravity and so the path of a ray of light can be bent by a large mass. This leads to an effect known as “Gravitational Lensing” whereby light from a more distant source is bent around a large mass – in this case a galaxy cluster – and the source appears, sometimes multiply, around the massive gravitational “lens”. Observations of such lensing enables the calculation of the mass of the lensing cluster. Once again, these measurements indicate that galaxy clusters have more mass than what is visible.

Note that some theorists still dispute the conclusion that Dark Matter is responsible for these gravitational effects. They suggest an alternative: that Newton’s laws of gravitation – one of the most well tested and established bedrocks of physics – need a little adjustment at large scales. This idea is called MOND – MOdified Newtonian Dynamics. Most physicists and astronomers do not view this favorably.

There are two possible candidate theories (which might both be correct…) as to what the dark matter could be: MACHOs or WIMPs – acronyms for MAssive Compact Halo Objects and Weakly Interacting Massive Particles, respectively. MACHOs are unseen large bodies that “live” in the halos of galaxies, i.e. surrounding the luminous part of galaxies. They could be something as “ordinary” as large planets that do not orbit a star or some sort of more “exotic” massive object. WIMPs, on the other hand, are some as yet unknown type of massive subatomic particles (massive compared to other subatomic particles, that is) that do not interact at all with light.

Dark matter has apparently played a crucial role in galaxies and clusters forming, by providing most of the mass that gravitationally pulled together the gas clouds from which galaxies formed. However only “luminous matter” was then able to “settle down” to “clump together” into the visible galaxies, by radiating away some of its (kinetic) energy.

Dark energy is even more strange. Recent observations over the past 20-30 years have measured the rate of expansion of the Universe, and have shown that it is accelerating. Teams led by Adam Riess, Saul Perlmutter and Brian Schmidt (who shared the 2011 Nobel Prize in physics for this) carefully measured the brightness of distant white dwarf supernovae – that all have the same intrinsic brightness at the source – that tell us how much the Universe has expanded and at what rate.

The acceleration must be caused by some energy or force causing space to accelerate its expansion. This was named “dark energy” as a parallel, but with no direct relation, to dark matter, or to being “dark”. (Most other kinds of energy are not “luminous” either, per se.) In fact, Einstein had predicted such an “energy” based on his theory of General Relativity – calling it the “Cosmological Constant” – but then rejected it. At the time he discovered general relativity, nobody knew of the expansion of space and the Universe was thought to be in a “steady state”. His “cosmological constant” was needed to balance his equation expressing general relativity against gravity causing the Universe’s collapse or expansion. When expansion of the Universe was discovered by Hubble over a decade later, he removed that factor from the equations expressing his theory:

Albert Einstein

with the Cosmological constant: Rμν – ½gμν = 8πGTμν + Λgμν

without the Cosmological constant: Rμν – ½gμν = 8πGTμν

It turns out Einstein was originally right after all, and that the extra factor provides for the acceleration of the expansion of space, but the evidence for that only came to light decades later.

It should be noted that locally, on the scale of galaxy clusters (or superclusters) the gravitational pull of the concentration of mass – mostly invisible dark matter – resists the expansion of the Universe as a whole and thus local space is not expanding.

These astronomical observations and theories can shed more light (yeah, pun intended…) on the blessing yotzer or – in particular on the “creates darkness” part of the phrase. (Other cosmological kavanah can be attached to the “forms light” part.)

Furthermore, we can now better interpret the phrase “Oseh shalom uboreh et hakol” – “who makes peace and creates all things”. We can understand shalom – peace – in this context to refer to “balance”.

“Dark energy” balances general relativity correctly explaining the expansion of the Universe. And the balance between dark matter and luminous matter and the mysterious “dark energy” that allowed the (local) Universe as we know it – with galaxies in which we can live – to form since the Big Bang. Dark matter is essential to formation of galaxies, and thus “hakol” – everything we know and see around us today – could not be created without there being the necessary proportion of dark matter, “choshesh”, as well as the luminous matter that light enables us to see.